An Adaptive Update-Based Cache Coherence Protocol for Reduction of Miss Rate and Traffic

Although directory-based write-invalidate cache coherence protocols have a potential to improve the performance of large-scale multiprocessors, coherence misses limit the processor utilization. Therefore, so called competitive-update protocols — hybrid protocols between write-invalidate and writeupdate — have been considered as a means to reduce the coherence miss rate and have been shown to be a better coherence policy for a wide range of applications. Unfortunately such protocols may cause high traffic peeks for applications with extensive use of migratory objects. These traffic peeks can offset the performance gain of a reduced miss rate if the network bandwidth is not sufficient. We propose in this study to extend a competitive-update protocol with a previously published adaptive mechanism that can dynamically detect migratory objects and reduce the coherence traffic they cause. Detailed architectural simulations based on five scientific and engineering applications show that this adaptive protocol can outperform a write-invalidate protocol by reducing the miss rate and bandwidth need by as much as 71% and 26%, respectively.